1. Field of the Invention
This invention relates to electric motors used to operate pump jacks and other devices with rotating or reciprocating masses.
2. Description of the Related Art
A pump jack is an above ground driving device for a reciprocating piston pump installed downhole in an oil well. The pump jack mechanically lifts liquid out of the well when there is not enough bottom hole pressure for the liquid to flow by itself to the surface. The pump jack is often powered by an electric motor that receives electrical power from an electric utility grid. A pump jack converts the rotary mechanism of the motor to a vertical reciprocating motion to drive the downhole pump. There are many different designs of pump jacks, including, but not limited to, conventional, the Lufkin Mark II, beam-balanced, air-balanced, slant hole and conventional portable. Pump jacks are available from many different suppliers, including Lufkin Industries, Inc. of Lufkin, Tex. and Cook Pump Company of Coffeyville, Kans.
The pump jack electric motor usually rotates a set of pulleys to a gear system or transmission, which in turn drives a pair of cranks or crank arms. For a typical conventional pump jack design, the cranks raise and lower an end of a lever or beam, known as a “walking beam,” that is pivoted on a sampson post or A-frame. A curved metal box known as a “horse head” is on the other end of the walking beam from where the crank arms are connected with the beam. A counterweight or reciprocating mass is typically attached to one end of the cranks. A pitman arm usually spans between the counterweight and the end of the walking beam opposite the horse head. A cable connects the horse head to a vertical polished rod, which is connected to the vertical string of tubulars or sucker rods running to the downhole pump.
The counterweight assists the motor in lifting the string of sucker rods or tubular string. When the motor lifts the counterweight upward, the horse head moves downward, pushing the sucker rods or tubular string downward. After the counterweight reaches the top of its rotation, it swings around and assists the motor to rotate the walking beam in the opposite direction using the counterweight's momentum and mass (kinetic energy). When the counterweight is free-falling downward from its uppermost position, the horse head moves upward, lifting the string of sucker rods upward. U.S. Pat. No. 4,051,736 proposes an improved pump jack for reciprocating an oil well pump.
Although there are different downhole pump designs, downhole pumps have traditionally comprised a plunger or piston reciprocating within a pump barrel located at or near the end of the production tubing. Two independent valves typically accomplish the pumping action. A standing check valve may be secured in the pump barrel beneath the piston, and the piston may include a traveling check valve. The upstroke of the piston opens the standing valve, and draws fluid into the pump barrel as the traveling valve remains closed. The downstroke of the piston opens the traveling valve and forces upward the fluid from the pump barrel as the standing barrel remains closed. U.S. Pat. Nos. 3,578,886; 4,173,451; and 6,904,973 propose downhole pumps.
It is well known that electric motors can enter an energy generation mode of operation. For an electric motor used with a pump jack, an energy generation mode can occur at any time during the rotation of the counterweight, depending on the condition of the balance between the counterweight and the tubular or rod string. The condition of the balance may fluctuate from pumping stroke to stroke, depending on the amount and composition of fluid being lifted by the rod string in each stroke. The polished rod and attached sucker rod or tubular string may be moving upwards or downwards in the energy generation mode.
A well owner must pay his electrical bill based upon the amount of power the pump jack motor consumes. The amount of energy consumed is measured by an energy meter. In the past, the amount of power consumed was measured by an analog electricity meter. Many digital electricity meters are now used. The energy meter, whether of analog or digital design, may be configured, at the discretion of the utility company, to allow or prevent crediting the customer for generated energy that is supplied back to the power grid. A pump jack system is such an inefficient generator that the quantity of consumed energy required to produce any generation significantly exceeds the generated energy. Therefore, regardless of whether the utility company credits generated energy, it is always beneficial to the customer to avoid energy generation.
During periods of generation, a motor will attempt to attain a voltage that exceeds the utility's line voltage, thereby causing current to flow in the opposite direction. The load provided by the utility grid serves as a brake, limiting the acceleration of the motor that would have otherwise occurred. This braking action of the motor prevents the falling weights of the pump jack from developing additional kinetic energy that might have assisted the pumping action. This converted kinetic energy could have served as an alternative to electrical energy from the utility grid.
In the past, engineers have tried unsuccessfully to save significant amounts of energy by turning off the pump jack electric motor during a portion of the pump jack cycle that may have included a period of generation. This has been attempted with various mechanical switches and relays. However, the parameters of the downhole pumps and wells vary over time, so these mechanical solutions have not worked.
Fluid flow in the well may vary as the well fills, and then “pumps off.” In some cases the volume of fluid pumped may change from one stroke to the next. The changing volumes, densities, viscosities, weights, and other properties of materials and/or fluids pumped, such as gas, oil, water, and slurry, may greatly alter the combined weight of the rod string and the column of fluid, thereby affecting the balance of the system and the demand on the motor. In some wells the tubular strings may be thousands of feet in length. The influx of different fluids into the well over time will significantly impact the operation of the motor.
With the introduction of the microprocessor, it became possible to turn off the electric motor by observing the current and voltage. However, the problem was in knowing when to turn the electric motor back on. Various open-loop fixed time delays were attempted in the past, but these attempts failed since the parameters of the downhole pumps and wells vary over time. Failure to turn the motor back on at the appropriate time can cause diminished energy savings and/or other undesirable effects.
When an AC induction motor is lightly loaded, reducing the voltage supplied will cause the motor to operate more efficiently, thereby saving energy. This is particularly evident in the case of single-phase motors, and to a lesser extent, three-phase motors. A large three-phase motor is usually naturally efficient at any load greater than approximately one-third of the motor's rated load.
The variation in the phase angle between the voltage applied to a motor and the current it draws bears an inverse relationship to the power drawn by the motor. A high phase angle indicates a lightly loaded motor, and a low phase angle indicates a heavily loaded motor.
The majority of pump jacks use three-phase motors as small as 5 horsepower (HP), but typically greater than 20 HP. These motors may be subjected to a periodically varying load, with the periods typically ranging from 5 seconds to 12 seconds. During a typical pumping stroke, the motor experiences a heavy load once or twice and a light load once or twice. Depending on the geometry and balancing of the pump jack, the falling weights (either the counterweights or the rod string) can force the motor beyond its synchronous speed, thereby causing it to behave as a generator. During such a time, the phase-angle exceeds 90 degrees.
U.S. Pat. No. 6,489,742 proposes a motor controller that includes power conveyance to an induction motor with a digital signal processor that calculates and optimizes supply of current for existent motor loading from a power supply and main voltage through a control element. Pub. No. U.S. 2010/0117588 proposes a motor controller and method for saving energy in an AC induction motor at every load wherein the motor is calibrated at two or more load points to establish a control line, which is then programmed into a non-volatile memory of the motor controller.
Pub. No. U.S. 2009/0046490 proposes an IGBT/FET-based energy savings device, system and method wherein a predetermined amount of voltage below a nominal line voltage and/or below a nominal appliance voltage is saved. Pub. No. U.S. 2009/0051344 proposes a TRIAC/SCR-based energy savings device, system and method wherein a predetermined amount of voltage below a nominal line voltage and/or below a nominal appliance voltage is saved. Pub. No. U.S. 2009/0200981 proposes a system and method for providing constant loading in AC power applications wherein at least one turn-on point of at least one half cycle of a modulating sine wave is determined, at least one turn-off point of the at least one half cycle of the modulating sine wave is determined, and at least one slice located between the at least one turn-on point and the at least one turn-off point in removed. Pub. No. U.S. 2010/0033155 proposes a power supply for IGBT/FET drivers that provides separated, isolated power to each IGBT/FET driver.
Proportional-integral-derivative (PID) control is a widely used technique applied to control algorithms and feedback mechanisms. A PID controller, as it is generally referred to, calculates a value based upon an “error.” Typically, the “error” is calculated as the difference between a measured process variable and a desired set point or target value. The PID controller attempts to minimize the error by adjusting the process control variables. In essence, the PID controller is a digital filter that has proportional, integral, and derivative parameters. The proportional value determines the reaction to the current error, the integral value determines the reaction based on the sum of the recent errors, and the derivative value determines the reaction based on the rate at which the error has been changing.
The above discussed U.S. Pat. Nos. 3,578,886; 4,051,736; 4,173,451; 6,489,742; and 6,904,973; and Pub. Nos. U.S. 2009/0046490; 2009/0051344; 2009/0200981; 2010/0033155; and 2010/0117588 are incorporated herein by reference for all purposes in their entirety.
A need exists to efficiently manage the energy usage of a pump jack electric motor, particularly during the energy generation mode. It would be desirable to substantially eliminate the energy generation mode if possible.